A subsea multiphase pump is an important part of subsea processing equipment used for subsea processing of well fluids from subsea oil and/or gas wells. A subsea multiphase pump comprises a variable speed electrical motor, such as a permanent magnet synchronous motor (PMSM).
Such motors are normally connected to a power source via a variable speed drive (VSD), where the purpose of the VSD is to control the motor to keep its torque at a desired level (often referred to as torque control). Hence, by means of the VSD, the torque is kept close to a torque reference value even if the load of the motor is changing.
The motor voltage and motor current are used by the VSD control algorithm in order to calculate the actual torque of the motor.
In subsea applications, the VSD is provided topside, often relatively close to the power source. Between the VSD and the motor a power transmission device is provided for transmission of power to the motor from the power source. In FIG. 1, this situation is illustrated schematically. Here the power transmission device is shown with a first transformer, a step-up transformer, connected topside between the VSD and a cable, and a second transformer, a step-down transformer, is connected subsea between the cable and the motor. This configuration, with a relatively long distance between the VSD and the motor, is often referred to as a long step-out configuration.
In FIG. 1, the transformers and cables are illustrated with their electrical equivalents comprising inductors, resistors and capacitors.
It is challenging to provide proper power supply and to control the subsea motor in such a configuration, due to the effect of the long cable and the transformers between the topside VSD and the subsea motor. Not only do these components introduce a voltage drop, resulting in a lower voltage reaching the motor than the one produced by the VSD, but more importantly they also cause a current loss or current gain on the motor side. These effects need to be compensated for, so that desirable mechanical performance (i.e. proper torque on the motor shaft) can be achieved.
The object of the invention is to improve the control of a subsea motor. In particular, the object of the invention is to improve the control of a subsea motor when a variable speed drive is used to supply power to an electric motor via a power transmission device.
Another object of the invention is to avoid or reduce dependency of subsea sensors, while still being able to operate the motor and to monitor the status of the motor.